1) Field of the Invention
This invention relates to non-volatile memory semiconductor devices and in particular, to an improved process for forming very narrow closely spaced buried bit lines.
2) Description of the Prior Art
In the quest to achieve microminiaturization of integrated circuit devices, individual elements have been made very small and the elements have been closely packed. Microminiaturization increases the speed of operation and reduces the production costs. In order to achieve this, each memory device must occupy less space on the device. Moreover, as read only memory (ROM's) devices are scaled down in size, there is a continuous challenge to produce narrower high density conductive lines (i.e., bit lines). In conventional methods for forming conductive lines, an insulating layer 12, (typically oxide) is deposited on a monocrystalline silicon substrate 10 as shown in FIGS. 1 through 3. Next, a masking layer 14, usually silicon nitride, is deposited on the insulating layer 12. Using conventional photolithography techniques, openings 16 that define the spaced, narrow bit lines are formed in the masking layer 14. Using layer 14 as an implant mask, impurity ions 20 are implanted through the openings 16 into the substrate 10 to form buried bit lines 22. The photolithography resolution capability determines the minimum line pitches 24 and line widths 23. The line pitch is the distance between on side of a line and the same side of an adjacent line. Next, the masking layer is etched away. A thin insulating layer 30, typically thermal oxide, is formed on substrate 10. Subsequently, a conducting layer 32, typically doped polysilicon, is deposited over the insulating layer. The following process steps to complete the read only memory (ROM) device are well known.
A major improvement in microminiaturized device fabrication is disclosed in U.S. Pat. No. 4,366,613. This patent discloses a spacer insulating layer on substantially vertical surfaces on a gate electrode of field effect transistors (FET's). This structure made possible the formation of dopant gradients in source and drain regions.